A disc brake to be used to brake a vehicle is structured such that a pair of pads are arranged across a rotor rotatable together with a wheel and, in braking, the two pads are pressed against the axial-direction two side surfaces of the rotor. The basic structure of such disc brake includes two kinds of structures, that is, a floating type structure and an opposed piston type structure. Of these two disc brakes, in the floating type disc brake, a caliper with a piston built therein on the inner side is axially shiftably supported on a support supporting the paired pads axially shiftably. In braking, the piston presses the inner side pad against the inner side surface of the rotor and, as a reaction of this, the caliper is shifted toward the inner side. And, a caliper pawl part formed in the outer side end of the caliper presses the outer side pad against the outer side surface of the rotor. Also, in the opposed piston type disc brake, multiple pistons are disposed on a caliper with a pair of pads axially shiftably supported thereon in such a manner that the pistons are arranged on the axial-direction two sides of the rotor. In braking, the pistons press the two pads against the axial-direction two side surfaces of the rotor. In each type, the two pads are formed by attaching linings to the front surface of a pressure plate having sufficient rigidity. And, in braking, the back surface of the pressure plate is pressed by the piston or the caliper pawl part, whereby the front surfaces of the linings and the axial-direction two side surfaces of the rotor are caused to rub each other.
Here, in this specification and Claims, terms [axial direction], [circumferential direction] and [radial direction] respectively mean the axial direction, circumferential direction and radial direction of the rotor in a state where the disc brake pad assembly is assembled to the disc brake, unless otherwise stated. Also, the term [peripheral edge] means the inner peripheral edge or outer peripheral edge of the rotor with respect to the radial direction thereof.
In the disc brake of each structure, in braking, the rotor is strongly held from the axial-direction two sides by the paired pads and is braked by a friction force applied to the contact portion between the linings constituting the two pads and the axial-direction two side surfaces of the rotor. In such braking, the portion to which such friction force is applied and the portions where the piston or the caliper pawl part press the two pads are shifted from each other along the axial direction by an amount equivalent to the thickness of the two pads; and, due to such shift, the attitudes of the two pads are easy to be unstable. When the attitudes of the two pads are unstable in braking, the behaviors of the two pads are hard to be smooth and thus they are vibrated, whereby there are produced noises called “squeak” and the uneven wear of the lining is easy to increase greatly.
In order to relieve such squeak and uneven wear, conventionally, it has been widely known to hold a shim plate between the back surface of a pressure plate constituting a pad and the leading end face of a piston or the inside surface of a caliper pawl part which operate as a pressure surface for pressing the back surface. Such shim plate has a single plate structure constituted of a single plate and, for enhancement in the relieving effect of the squeak and uneven wear, there has been also widely employed a two-plate structure in which an inside shim plate and an outside shim plate are superimposed on each other. Also, whether a single plate structure or a two-plate structure, locking pieces formed in the multiple locations of the inside and outside two peripheral edges of the shim plate are engaged with the inside and outside two peripheral edges of the pressure plate, whereby the shim plate is supported on the back surface side of the pressure plate.
The patent document 1 discloses, as a disc brake pad assembly, a structure as shown in FIGS. 12˜14. In this conventional structure, a shim plate 5 constituted of an inside shim plate 3 and an outside shim plate 4 is mounted on the back surface of a pressure plate 2 constituting a pad 1. In the pad 1, a lining 6 is fixedly attached to the front surface (the surface to be opposed to the side surface of the rotor when the pad 1 is assembled to the disc brake) of the pressure plate 2 by such a large connecting force that can prevent it from being shifted due to a brake torque applied thereto in braking. The inside shim plate 3 is made of a metal plate such as a stainless steel plate and includes a flat plate-shaped inside main body 7 and multiple inside locking pieces 8a, 8b, 8c. Also, the inside main body 7 includes multiple open holes 9, 9 used to hold grease therein. Also, of the inside and outside two peripheral edges of the pressure plate 2, the outer pressure edge includes a locking recess 10 in the circumferential direction central part thereof, while the inner peripheral edge includes a pair of steps 11, 11 formed near to the circumferential direction two ends thereof. While, of the inside locking pieces 8a, 8b, 8c of the inside shim plate 3, the inside locking piece 8a on the outside diameter side is engaged with the locking recess 10 and the inside locking pieces 8b, 8c on the inside diameter side are engaged with the two steps 11, 11, the pressure plate 2 is sandwiched from the radial-direction two sides thereof by the inside locking pieces 8a, 8b, 8c. In this state, the inside shim plate 3 is mounted on the back side of the pressure plate 2 in such a manner that it is restricted (substantially, prevented) from shifting in the peripheral and radial directions thereof.
Also, the outside shim plate 4 is made of a metal plate such as a stainless steel plate and includes a flat plate-shaped outside main body 12 and multiple outside locking pieces 13a, 13b, 13c. In this outside shim plate 4, while the outside locking pieces 13a, 13b, 13c are superimposed on the inside locking pieces 8a, 8b, 8c respectively, the outside main body 12 is superimposed on the inside main body 7. In this state, the outside shim plate 4 is assembled to the inside shim plate 3 in such a manner that it can shift in the circumferential direction. Thus, the circumferential direction width dimension of the outside locking piece 13a is set smaller than those of the locking recess 10 and the inside locking piece 8a, while the distance between the mutually opposed circumferential direction outside edges of the outside locking pieces 13b and 13c is set smaller than the distance between the two steps 11 and 11.
Here, it has also been conventionally proposed to use the disc brake pad assembly having the above structure while it is incorporated into an electric disc brake such as a disc brake equipped with an electric parking mechanism. In this electric disc brake, there has been widely employed a structure in which a piston fitted in a cylinder is pushed out toward a pad arranged on the inner side by a conversion mechanism such as a feed screw mechanism for converting the rotation motion of an electric motor to linear motion. Also, to stop the rotation of the piston and receive a rotation force transmitted to the piston by the conversion mechanism, there has been used a technology that a recess is formed in the leading end face of the piston and a dowel formed on the back surface of the pressure plate of a pad arranged on the inner side is engaged into the recess. Thus, when the disc brake pad assembly having the above structure is used while it is incorporated into the electric disc brake, a notch is formed in a portion of a shim plate covering the back surface of the pressure plate, whereby the dowel formed on the back surface of the pressure plate is engaged into the recess formed in the leading end face of the piston.
Meanwhile, when the above-mentioned dowel and recess rotation preventive mechanism is employed, for reason of complicated control of the electric motor or other, the inventors et al. have pushed forward a study of a structure which omits the dowel and recess but uses a friction force or the like acting on the piston to stop rotation thereof. And, in this case, even when the disc brake pad assembly is used, the rotation stop structure using the dowel and recess is not provided but there is proposed a structure that the leading end face of the piston is contacted with the back surface of a shim plate constituting the disc brake pad assembly. However, when this structure is employed simply, there is a possibility that the following problems can occur.
That is, as shown in FIG. 14, in braking, a rotation force is transmitted through the leading end face of a piston 14 to the back surface of an outside shim plate 4 against which the leading end face of a piston 14 is pressed. And, in a disc brake pad assembly of a conventional structure, such rotation force is received by any one of engaged portions between the outside locking pieces 13a, 13b, 13c provided on the peripheral edge of the outside shim plate 4 and the peripheral edge (locking recess 10 and steps 11, 11) of the pressure plate 2. And, the outside shim plate 4 is restricted from rotating relative to the pressure plate 2 about the axis O of the piston 14. Thus, excessive stress is applied to the outside locking pieces 13a, 13b, 13c, thereby raising a possibility that they can be plastically deformed or can be damaged, for example, can be broken through long use.